Method for surface treatment of magnesium or magnesium alloy by anodization

ABSTRACT

Disclosed herein is a method for the surface treatment of magnesium or a magnesium alloy by anodization to form an anodized oxide coating on the magnesium or magnesium alloy. The method comprises: removing impurities and an oxide layer present on the surface of magnesium or a magnesium alloy using a strongly alkaline aqueous solution (pretreatment); and immersing the pretreated magnesium or magnesium alloy in an alkaline electrolyte and applying a direct current having a current density of 3 A/dm 2  or less to the electrolyte to form a magnesium oxide coating (microarc plasma anodization).

BACKGROUND

1. Technical Field

The present invention relates to a method for the surface treatment of magnesium or a magnesium alloy by anodization to improve the corrosion resistance of the magnesium or magnesium alloy. More specifically, the present invention relates to a method for the surface treatment of magnesium or a magnesium alloy using an environmentally friendly electrolyte without the need to use environmental contaminants, such as chromium and manganese, to basically prevent the occurrence of toxic waste water and improve the corrosion resistance and coating adhesiveness of the magnesium or magnesium alloy as an undercoat.

2. Description of the Related Art

It is known that magnesium is the sixth most abundant metal element and has about two thirds the specific gravity of aluminum. Magnesium is the lightest metal of currently available metals and has advantages in terms of specific strength (i.e., strength/specific gravity), durability, electromagnetic shielding properties, vibration absorption, etc. Based on these advantages, magnesium alloys are currently being used or being considered to use in a wide range of applications, including automotive parts and components (e.g., housings, oil fans and seat frames), portable notebook computers, cellular phone cases, leisure and sports goods, and high-tech spacecraft materials.

Despite the above advantages, magnesium alloys are sensitive to both alkalis and acids owing to their high activation tendency. Accordingly, magnesium or a magnesium alloy suffers from severe galvanic corrosion when exposed to corrosive environments. This requires the formation of a highly corrosion-resistant organic or inorganic coating on the surface of the magnesium or magnesium alloy.

Surface treatment methods of magnesium alloys are classified into dry coating and wet coating processes. The dry coating process has many limitations in that deposition plating under vacuum is difficult to perform owing to high vapor pressure, the working space is limited and considerable production costs are incurred.

According to the wet coating process, in the course of surface coating of a magnesium alloy by electroplating or electroless plating, high activation tendency of the magnesium alloy induces substitution of the magnesium ions with ions present in an electrolyte on the surface of the magnesium alloy. This substitution promotes the degree of aging of the electrolyte and the galvanic corrosion cause poor corrosion resistance of the plated surface of the magnesium alloy, leading to low value-added production.

In view of the above problems, conversion coating processes have been developed as alternative surface treatment methods of magnesium alloys. Such conversion coating processes are typically divided into chromate conversion coating and non-chromate conversion coating processes. The conversion coating processes do not give satisfactory corrosion and wear resistance of coatings, limiting their application to the pretreatment of coatings. The conversion coating processes use chromium (Cr) or manganese (Mn), which is known to cause serious pollution problems, for the formation of coating. Thus, the conversion coating processes have a decisive disadvantage in that there is a high possibility of regulating the production of coatings in the near future.

Anodization is known as a process for the coating of a magnesium alloy. A great deal of research on galvanic anodizing, HAE anodizing and Dow 17 anodizing has been conducted.

There are differences between amorphous anodization of aluminum (Al) and its alloys and anodization of magnesium and its alloys in terms of the pH and kind of electrolytes for forming oxide coatings and the reaction principle. According to the anodization of magnesium and its alloys, the expansion rate of an oxide coating formed by the oxidation of magnesium is lowered such that the anodized coating contains many pores, causing a low degree of compaction. In this connection, the corrosion resistance of the coating deteriorates. The anodization causes pollution problems because it uses chromium or manganese, likewise other coating processes. FIG. 1 is a flow chart illustrating a prior art surface treatment method for forming an anodized coating on the surface of magnesium or a magnesium alloy. Referring to FIG. 1, the anodization process includes a series of degreasing the surface of the magnesium or magnesium alloy, water washing, removal of an oxide layer, water washing, anodization, water washing and drying.

In attempts to solve the problems of the prior art, numerous modified techniques, called plasma anodization and microarc anodization, have been developed. These techniques share the use of electrolytes containing Si4+ ions in common. The electrolytes serve to increase the intensity of surface arc at a high voltage (150-500 V). The Si4+ ions form magnesium silicate to fill internal pores of the coatings, making the coating texture dense. However, since the high-voltage anodization requires the use of a rectifier having a very high capacity, it is not appropriate for the mass production of magnesium products. Further, many cracks are formed in the oxide coatings made of hard and inflexible ceramic. Further, the oxide coatings show poor adhesion to coatings formed in the subsequent step to improve the corrosion resistance. Alternative efforts have been made to lower the high voltage by using direct/alternating current overlapping or pulse power supply method. However, the commercial effects are not great despite the use of high-priced rectifiers.

Sandblasting may be used as a pretreatment step for anodization. In this case, the removal of impurities (such as oils) present on the surface of magnesium is not sufficient, which may induce defects of the anodized coating. Trichloroethylene (TCE) or acetone as an organic solvent may be used. In this case, special care should be taken because the organic solvent causes serious pollution.

BRIEF SUMMARY

The present invention has been made in an effort to solve the problems of the prior art surface treatment methods, and it is an object of the present invention to provide a technique for forming an anodized coating with good characteristics on the surface of magnesium or a magnesium alloy as a base material through the improvement of pretreatment processing and by microarc plasma anodization wherein a low voltage and a direct current are applied in an environmentally friendly electrolyte.

It is another object of the present invention to provide a technique for forming an anodized coating with good characteristics by improving the pretreatment processing to solve the problems of poor degreasing and greatly decrease the amount of organic waste water and by microarc plasma anodization wherein a low voltage and a direct current are applied in an environmentally friendly electrolyte containing no chromium, manganese and Si4+ ions.

According to an aspect of the present invention, there is provided a method for the surface treatment of magnesium or a magnesium alloy by anodization to form an oxide coating on the magnesium or magnesium alloy, the method comprising: removing impurities and an oxide layer present on the surface of magnesium or a magnesium alloy using a strongly alkaline aqueous solution (pretreatment); and immersing the pretreated magnesium or magnesium alloy in an alkaline electrolyte containing no Si4+ ions and applying a direct current having a current density of 3 A/dm2 or less to the electrolyte to form a magnesium oxide coating (microarc plasma anodization).

In an embodiment, the method further comprises, after the microarc plasma anodization, washing the magnesium or magnesium alloy, on which the coating is formed, with distilled water, followed by drying with hot air (post-treatment).

In a preferred embodiment, the strongly alkaline aqueous solution used in the pretreatment step contains 30-60 g/L of KOH, 150-200 g/L of NaOH, 20-30 g/L of an amine oxide, 5-10 g/L of an α-alkoxyisobutyric acid alkyl ester and a wetting agent.

In a preferred embodiment, the alkaline electrolyte used in the microarc plasma anodization step contains 20-150 g/L of potassium hydroxide (KOH), 20-150 g/L of sodium carbonate (Na₂CO₃), 5-40 g/L of trisodium citrate, 3-40 g/L of potassium fluoride (KF), and 3-40 g/L of ammonium bifluoride (NH₄HF₂). If the alkaline electrolyte is out of the composition defined above, the formation of the anodized coating on the surface of the base material is excessively retarded or the burning of the coating surface occurs, posing the danger that the coating may be inhomogeneous (see Table 2).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a flow chart illustrating a prior art surface treatment method for forming an anodized coating on the surface of magnesium or a magnesium alloy;

FIG. 2 is a flow chart illustrating a surface treatment method for forming an anodized coating on the surface of magnesium or a magnesium alloy according to an embodiment of the present invention; and

FIG. 3 is a magnification image showing the surface texture of a magnesium alloy plate whose surface is treated by the method illustrated in FIG. 2.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will now be described in greater detail with reference to the accompanying drawings. These embodiments are provided so that this disclosure will fully convey the scope of the invention to those skilled in the art. Accordingly, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

FIG. 2 is a flow chart illustrating a surface treatment method by anodization according to the present invention. Referring to FIG. 2, the method of the present invention comprises alkaline degreasing (S10), microarc plasma anodization (S20) and drying (S30). Water washing (S12) is performed between the alkaline degreasing (S10) and the microarc plasma anodization (S20). Another water washing (S22) is performed between the microarc plasma anodization (S20) and the drying (S30).

In the step of alkaline degreasing (S10), magnesium or a magnesium alloy as a base material is immersed in a strongly alkaline aqueous solution to remove impurities remaining on the surface of the base material while dissolving organic materials strongly attached to the surface of the base material by chemical reactions (degreasing). The degreased base material is washed with water (S12), followed by microarc plasma anodization (S20) to form a magnesium oxide coating containing magnesium carbonate on the surface of the base material. Next, the anodized coating is washed with water (S22) and dried (S30).

In the alkaline degreasing step (S10), processing oils and release agents attached to the surface of the base material are emulsified by an amine oxide and alkali components. The emulsified oils are dispersed with an α-alkoxyisobutyric acid alkyl ester and are dissolved in the water. This procedure makes the surface of the base material clean. More specifically, the alkaline degreasing step is carried out in such a manner that the magnesium alloy is immersed in a strongly alkaline aqueous solution containing 30-60 g/L of KOH, 150-200 g/L of NaOH, 20-30 g/L of an amine oxide, 5-10 g/L of an α-alkoxyisobutyric acid alkyl ester and a wetting agent at about 35 to about 50° C. If the alkaline aqueous solution is out of the composition defined above, the removal of oily components present on the surface of the base material may be too retarded or the surface of the base material may be discolored. Therefore, it is desirable to degrease the surface of the base material using the alkaline aqueous solution having the above composition.

In the microarc plasma anodization step (S20), the pretreated magnesium product is immersed in an environmentally friendly electrolyte containing potassium hydroxide and sodium carbonate as main components, and then a direct current having a current density of 3 A/dm² or less is applied thereto.

The environmentally friendly electrolyte is an alkaline electrolyte having a pH ranging from 9 to 14. The components of the alkaline electrolyte and the concentrations thereof are shown in Table 1. Coloring may be done simultaneously with the anodization. One or more additives may be added to the alkaline electrolyte depending on the composition of the magnesium product.

TABLE 1 Component Concentration (g/L) Treatment conditions Potassium 20-150 Treatment Temp.: 20-55° C. hydroxide (KOH) Sodium carbonate 20-150 Current density ≦3 A/dm² (Na₂CO₃) Trisodium citrate 5-40 pH: 9.0-14.0 Potassium 3-40 Arc voltage: 45-70 V fluoride (KF) Ammonium 3-40 bifluoride (NH₄HF₂)

The environmentally friendly electrolyte contains no manganese or chromium. Particularly, the presence of a large amount of the sodium carbonate (Na₂CO₃) in the electrolyte composition enables filling of an insoluble and more compact magnesium carbonate coating in the less compact magnesium oxide coating formed by the microarc plasma anodization. In addition, the reaction of the magnesium carbonate with the trisodium citrate leaves bumps on the surface of the coating. The bumps play an important role in improving the adhesiveness of a coating to be formed after the anodization.

FIG. 3 is a photograph showing the dense texture of the magnesium oxide coating after the microarc anodization using the electrolyte containing sodium carbonate and the bumps formed on the surface of the magnesium oxide coating.

The coating is formed at a very high rate because the electrolyte has a current efficiency of 100%. Accordingly, the microarc plasma anodization can be performed even at a current density as low as 3 A/dm². In addition, since the oxide coating is formed even at a low voltage of 45 to 70 V for the formation of microarc, the capacity of a rectifier does not cause any problem in mass production.

The microarc plasma anodization was performed on magnesium alloy plates (AZ31) using respective electrolytes having the compositions indicated in Table 2 for 20 min. The anodized magnesium alloy plates were thoroughly washed with distilled water, dried with hot air at 120° C. for 5 min and cooled. A salt spray test was conducted on the anodized magnesium alloy plates, and the results are shown in Table 2.

TABLE 2 Ammonium bifluoride Trisodium Arc Salt spray Component Na₂CO₃ (NH₄HF₂) KF KOH citrate anodization test (48 hr) 1 15 7.5 7.5 50 10 X¹⁾ 0 2 20 7.5 7.5 50 10 ◯²⁾ 9.1 3 150 7.5 7.5 50 10 ◯ 9.5 4 160 7.5 7.5 50 10 Δ³⁾ 4.3 5 50 7.5 3 50 10 X 0 6 50 7.5 5 50 10 ◯ 10.0 7 50 7.5 40 50 10 ◯ 9.5 8 50 7.5 45 50 10 Δ 7.8 9 50 3 7.5 50 10 X 0 10 50 4 7.5 50 10 ◯ 9.2 11 50 40 7.5 50 10 ◯ 9.5 12 50 45 7.5 50 10 Δ 6.5 13 50 7.5 7.5 15 10 X 0 14 50 7.5 7.5 20 10 ◯ 9.8 15 50 7.5 7.5 150 10 ◯ 9.5 16 50 7.5 7.5 160 10 Δ 2.5 17 50 7.5 7.5 50 4 X 0 18 50 7.5 7.5 50 5 ◯ 9.1 19 50 7.5 7.5 50 40 ◯ 10.0 20 50 7.5 7.5 50 45 Δ 2.1 Note: ¹⁾No arc formed, ²⁾Arc formed, ³⁾Unstable arc formed

In a state where no additional coating was formed, the coatings formed under the microarc plasma anodization conditions defined in the present invention showed good corrosion resistance without any defects for 48 hr after the anodization.

A coating was formed on each of the magnesium alloy plates (AZ31) anodized for 10 min. No corrosion was observed in the coated magnesium alloy plates for 240 hr after the salt spray test. From these results, it can be seen that the coatings formed by the anodization for about 10 min can be used as undercoats having sufficient adhesiveness.

In conclusion, the anodization surface treatment method of the present invention is suitable for the formation of an undercoat of a magnesium alloy. More specifically, according to the method of the present invention, silicate components, the electrolyte does not contain any silicate component, which causes turbidity of the electrolyte and clogging of a plate heat exchanger. In addition, when a direct current having a constant current density is applied to the magnesium alloy material as an anode in the aqueous solution to induce electrolytic reactions, the voltage increases at a constant rate in the early stage. Thereafter, when the voltage reaches 45-70 V, microarc plasma is generated on the surface of the magnesium alloy immersed in the aqueous solution by dielectric breakdown to form a grayish white magnesium oxide coating containing many holes on the surface thereof. When a coating is formed on the magnesium oxide coating in the subsequent processing step, the holes of the magnesium oxide coating greatly improve the adhesion of the coating to the magnesium oxide coating, achieving excellent corrosion and wear resistance.

According to the method of the present invention, the pretreatment step before the anodization step includes degreasing using an environmentally friendly strongly alkaline aqueous solution, unlike a conventional pretreatment step using an organic solvent such as trichloroethylene (TCE) or acetone or sandblasting whose degreasing effect is small. The pretreatment step is simple and keeps the surface of the base material, on which an anodized coating is to be formed, unchanged, thus minimizing the defective proportion of final products. In addition, the method of the present invention basically inhibits the occurrence of organic waste water or magnesium dust during sandblasting in the conventional pretreatment process.

Furthermore, the microarc plasma anodization step using the electrolyte, which is environmentally friendly and easy to purchase, instead of an electrolyte containing environmental contaminants, such as chromium and/or manganese, basically inhibits the occurrence of waste water.

Moreover, the microarc plasma anodization step using the electrolyte containing sodium carbonate (Na₂CO₃) enables the formation of a compact anodized magnesium oxide coating, compared to the prior art, and can form fine and uniform bumpy magnesium compounds on the surface of the anodized coating, thus maximizing the adhesiveness of a coating formed in the subsequent step and achieving improved corrosion resistance, wear resistance and coatability.

The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

1. A method for the surface treatment of magnesium or a magnesium alloy by anodization to form an oxide coating on the magnesium or magnesium alloy, the method comprising: removing impurities and an oxide layer present on the surface of magnesium or a magnesium alloy using a strongly alkaline aqueous solution and thereby forming a pretreated magnesium or magnesium alloy; and immersing the pretreated magnesium or magnesium alloy in an alkaline electrolyte and applying a direct current having a current density of 3 A/dm² or less to the electrolyte to form a magnesium oxide coating through microarc plasma anodization.
 2. The method of claim 1, further comprising, after the microarc plasma anodization, washing the magnesium or magnesium alloy, on which the coating is formed, with distilled water, followed by drying with hot air.
 3. The method of claim 1, wherein the strongly alkaline aqueous solution used in the pretreatment step comprises 30-60 g/L of KOH, 150-200 g/L of NaOH, 20-30 g/L of an amine oxide, 5-10 g/L of an α-alkoxyisobutyric acid alkyl ester and a wetting agent.
 4. The method of claim 1, wherein the alkaline electrolyte used in the immersing step comprises sodium carbonate (Na₂CO₃).
 5. The method of claim 1, wherein the alkaline electrolyte used in the immersing step comprises 20-150 g/L of potassium hydroxide (KOH), 20-150 g/L of sodium carbonate (Na₂CO₃), 5-40 g/L of trisodium citrate, 3-40 g/L of potassium fluoride (KF), and 3-40 g/L of ammonium bifluoride (NH₄HF₂). 